Thermodynamics, kinetics and isothermal studies of tartrazine adsorption onto microcline/MWCNTs nanocomposite and the regeneration potentials

The quest for a cheap, effective, and eco-friendly wastewater treatment technique that is free of secondary toxic byproducts, calls for the fabrication of a nature-friendly adsorbent with a robust capacity to decontaminate polluted water sources and be recycled. To this end, we report the fabrication of novel nanocomposite (KMCM) from microcline (KMC) and multiwall carbon nanotubes (MWCNTs). The adsorbents (KMC and KMCM) were characterized using XRD, BET, SEM, TGA and FTIR. The novel and low-cost nano sorbent were designed for the elimination of tartrazine (Tatz) from wastewater. The adsorption of Tatz onto KMC and KMCM was influenced by adsorbent dose, initial Tatz concentration, contact time and solution pH. Experimental data acquired from the equilibrium studies were well addressed by the Langmuir isotherm model. The maximum uptake capacity of 37.96 mg g−1 and 67.17 mg g−1 were estimated for KMC and KMCM. The kinetics for the adsorption of Tatz onto KMC and KMCM was best expressed by pseudo-second-order and Elovich models. The thermodynamic parameters revealed that the uptake of Tatz onto KMC and KMCM was an endothermic (ΔH: KMC = 35.0 kJ mol−1 and KMCM = 42.91 kJ mol−1), entropy-driven (ΔS: KMC = 177.6 J K−1 mol−1 and KMCM = 214.2 J K−1 mol−1) and spontaneous process. Meanwhile, KMCM demonstrated good reusability potential and superior adsorption efficiency when compared to other adsorbents.

Batch adsorption. The capacity of KMC and KMCM to remove Tatz from the aqueous solution was investigated using the batch adsorption technique. 1 g of Tatz powder was weighed into 1 dm 3 of deionized water to prepare the stock solution (1000 mg dm −3 ). The working solution (100 mg dm −3 ) was prepared from the stock solution via serial dilution and adjusted to desired solution pH using 0.1 mol dm −3 NaOH, or 0.1 mol dm −3 HCl solutions. The batch adsorption of Tatz was achieved by contacting KMC or KMCM (0.05 g, 150 rpm) with Tatz solution (100 mg dm −3 , 25 cm 3 ) in a 100 cm 3 stoppered glass bottle for 1440 min at room temperature. The mixture was filtered, and the equilibrium concentration of Tatz was determined using ultraviolet-visible (UV-vis) spectrophotometry (λ max = 426 nm) 62 . The implication of dosage, contact time, solution temperature, solution pH and initial adsorbate concentration were assessed for the optimization of the removal process. Meanwhile, the uptake capacity (see Eq. S1), removal efficiency (see Eq. S2) 63 , kinetics (see Table S1), and isotherm (see Table S2) analysis of Tatz adsorption onto KMC and KMCM were determined as described in the supplementary information S1.

Reusability.
To demonstrate the reusability of the KMC and KMCM for the removal of Tatz, aqueous ethanol was used as the eluting agent. Briefly, 0.5 g of KMC or KMCM was contacted with 250 cm 3 of 100 mg dm −3 Tatz solution and shaken for 1440 min at room temperature to adsorb Tatz. The mixture was filtered, washed, and dried. 0.05 g of the spent adsorbents (KMC-Tatz or KMCM-Tatz) were then eluted using 25 cm 3  www.nature.com/scientificreports/ ethanol for 30 min at room temperature. The adsorption efficiency of KMC and KMCM was calculated using Eq. (S2) describes in the supplementary information S1.

Determination of pH point of zero charge. The pH at the point of zero charge (pH PZC ) of KMC and
KMCM is a vital factor in unfolding the ionization behaviour of these novel adsorbents. The pH PZC of KMC and KMCM was achieved using the previously described method 64 . 50 cm 3 of 0.01 M NaCl was measured into eleven conical flasks and adjusted to pH 2-12. To each of the conical flasks, 0.1 g of KMC or KMCM was contacted and allowed to stand for 48 h. The final pH of each mixture was determined and a plot of the final pH versus the initial pH was obtained from which the pH PZC of KMC and KMCM was deduced from the line intercept.

Results and discussion
The surface morphology of KMC, KMC-Tatz, KMCM and KMCM-Tatz were visualized using FESEM spectroscopic technique. The result of FESEM analysis was displayed in Fig. 1. The external micro-graphical structure of the KMC, showed clusters of irregularly shaped particles embedded on a sheetlike surface having a homogenous texture (see Fig. 1). Meanwhile, after the adsorption of Tatz onto KMC, the KMC-Tatz micrograph was noticed to have a smooth surface with ultrathin fragments of sheets in layers, suggesting that the surface of KMC was covered with Tatz (see Fig. 1). On the other hand, the micrograph acquired for KMCM and KMCM-Tatz exhibited heterogonous characteristics consisting of a weblike long cylindrical structure on sheets of irregular shapes. However, KMCM-Tatz tends to sustain a non-porous and smooth surface, indicating the coverage of KMCM by tartrazine molecules. The functional groups on the surface of KMC and KMCM were investigated by making use of the FTIR spectrophotometer. Figure 2, showed the compared spectra of the Tatz-loaded and unloaded adsorbents. Characteristic peaks of microcline were observed at 467 cm −1 , 586 cm −1 , 640 cm −1 , and 1003 cm −1 and were attributed to -Si-O asymmetrical bending, O-Si-(Al)-O symmetrical bending, Al-O co-ordination, and Si-(Al)-O stretching vibrations 65 . The intensity of the KMC peaks was noticed to reduce on the FTIR spectra acquired for the nanocomposite (KMCM). This could be attributed to the masking effect of the modifier (f-MWCNTs). Meanwhile, after the modification step, a broad peak at 3425 cm −1 was observed for the pristine nanocomposite (KMCM) and this was attributed to the vibration of -O-H and -NH bonds arising from the functionalization  XRD characterization. The structure of KMC was identified by making use of X-ray diffraction spectra acquired within the 2theta range of 5.0-80° (see Fig. 3). The spectra revealed that the mineral (KMC) contained a varied percentage of quartz, muscovite, albite, and microcline. The diffraction peaks of KMC are in good agreement with the reference library ICDD 19-926. Meanwhile, the XRD diffractogram of the microcline used for the adsorption of methylene blue showed similar identical diffraction peaks with our report 66 . On the other hand, the X-ray diffraction spectra obtained for KMCM showed no significant change with respect to the 2Theta values when compared to KMC. However, the reduced intensity of the peaks and formation of new peaks at 2Theta 23.43, 43.24 and 54.07 was observed and were attributed to the reflections of graphite from MWCNTs respectively (ICDD No. 01-074-2379). This reflects the homogeneity and the consistency in the crystallinity of the nanocomposite.  www.nature.com/scientificreports/ Thermal analysis. The thermal stability and behaviour of KMC and KMCM were validated by making use of the thermo-gravimetric analysis (TGA). Weight loss below 200 °C is attributed to the loss of hygroscopic water from the constituent of KMC and KMCM (see Fig. 4). Meanwhile, the thermogram of KMC revealed the consistent loss of mass from 200 to 800 °C, this could be associated with the loss of constitutional water resulting from the evolution of hydroxides within the quartz, albite, and muscovite network in KMC 67 . Within the investigated temperature range, KMCM was noticed to have a three-stage degradation phase. This could be attributed to surface-bound water, volatile inorganic constituents of the nanocomposite and amorphous carbon (MWCNTs). It was observed that about 17.39 wt% (KMC) and 39.74 wt% (KMCM) were degraded over the range of temperature (25-800 °C) employed (see Fig. 4). This shows the implication of the modification step and the robust thermal stability of KMCM. The specific surface areas and pore size distribution of the adsorbents (KMC and KMCM) were evaluated from the N 2 sorption-desorption isotherms techniques at 77 K. From Fig. 5, it can be observed that the specific surface area of KMCM (10.77 m 2 /g) was larger than that of KMC (3.223 m 2 /g) (see Table 1). Meanwhile, a similar result was observed from the pore size and pore volume estimation made by making use of the Barrett-Joyner-Halenda (BJH) approach. It shows that the modification of the microcline rock (KMC) with f-MWCNTs enhanced the surface area of the nanocomposite, which further justifies the higher uptake of the composites (KMCM) due to an increase in the available active sites (see Fig. 5) The isotherm acquired from the adsorption-desorption of N 2 by the adsorbents were observed to correspond to type II with H3 hysteresis loop of the IUPAC classification.   Fig. 7). This shows that at pH higher and lower than these values (5.6 (KMC) and 4.3 (KMCM)), the surface of the KMC and KMCM will be negatively and positively charged respectively. This suggests that at pH 2, Tatz may exist in their monomeric forms and this could enhance, easy pore capture of the Tatz molecules, but as the solution pH increases beyond pH 2, the -OH groups increases and may repel the sticking of the Tatz to the surface of KMC and KMCM. Hence, hydrophobic interaction and Tatz entrapment in the pores of the nanocomposites may be the mechanism responsible for Tatz adsorption onto the surface of KMC and KMCM. Our results are in good agreement with the report of Tatz adsorption onto other adsorbents 30 .  www.nature.com/scientificreports/ Kinetics study. The implication of agitation time on the adsorption of Tatz was investigated by varying the agitation time from 5 to 1440 min. The outcome of this study showed that the uptake of Tatz by KMC and KMCM was in two phases (see Fig. 8). These phases include the fast and the slow phase, similar behaviour has been reported for Tatz removal by surfactant-ionic liquid bi-functionalization of chitosan beads 68 and zincaluminum layered double hydroxide 30 . The elimination of Tatz by KMC and KMCM were optimum at 120 and 180 min respectively, above these optimum contact times, an insignificant amount of the Tatz was removed. However, to ensure equilibrium attainment, 1440 min was used for further experiments. The fast phase may be due to the availability of sufficient active sites on the surfaces of KMC and KMCM at the early stage of the adsorption. Meanwhile, the slow phase may probably be due to insufficient active sites on the surface of the adsorbents or as a result of pore entrapment of the Tatz molecules. To better understand the mechanism responsible for the removal of Tatz from the simulated wastewater, experimental data acquired from the time-dependent uptake study for Tatz removal by KMC and KMCM were fitted into four mathematical empirical kinetics models namely pseudo-first-order, pseudo-second-order, Elovich and Weber-Morris intraparticle diffusion via the non-linear least square analysis (nlls) (see Fig. 9). The least sum of squared residuals (SSR) and residual square errors (RSE) of the different kinetic models were compared and the model with the least values was selected as the model that best describes the mechanism accountable for the elimination of Tatz over the studied duration. As displayed in Table 2, the uptake of Tatz by KMC was best expressed by pseudo-second-order kinetics. On the other hand, Elovich kinetic model was observed to best describe the kinetics involved in the removal of Tatz by  www.nature.com/scientificreports/ KMCM. Pseudo-second-order kinetics suggest that the elimination of Tatz by KMC is a chemisorption process involving a bimolecular sorbate-sorbent adsorptive interaction. Our results corroborate with the uptake of Tatz onto Katira gum-cl-poly(acrylic acid-co-N-vinyl imidazole) hydrogel 69 and polyamidoamine dendrimer gel 70 . Meanwhile, the adsorption of Tatz by KMCM is predominantly a chemisorption process. This suggests that the elimination of Tatz by both sorbents involves sharing or exchange of electrons. The mechanistic kinetics model (intraparticle diffusion concept) was used to assess the possible steps involved in the transfer of Tart molecules from the bulk solution into pores of KMC and KMCM as well as external surface adsorption. The intraparticle diffusion rate constant for the uptake of Tatz onto KMC and KMCM was estimated from the plot of the square root of time (t 1/2 ) against the uptake capacity of the KMC and KMCM. From the plot, a linear relationship with the line passing through the origin of the graph indicates that the rate-determining step is intraparticle diffusion. On the contrary, non-zero intercepts were extrapolated from the plots, and this indicates a difference in mass transfer rate between the early and late stages of the adsorption process. It also indicates that intraparticle diffusion may not be the sole rate-determining step in the adsorption of Tatz onto KMC and KMCM.
Effect of adsorbents dosage. By changing the sorbent mass from 0.01 to 0.4 g, the effect of the adsorbent dose on the removal of Tatz from the aqueous was examined. An increase in the uptake efficiency of KMC and KMCM was observed with an increase in the adsorbent dose (see Fig. 10). This could be attributed to the www.nature.com/scientificreports/ increasing number of chemical moieties per adsorbent unit with the increase in dosage 71 . The sorbate-to-sorbent ratio was noticed to decrease with an increase in the dosage of KMC and KMCM (see Fig. 10). This phenomenon could be due to the disproportional increase of the active sites on the sorbent surface and sorbent mass 72 , it could also be due to an increase in the active sorbent mass with fixed sorbate concentration. At the end of this study, 0.05 g was selected as the optimum mass for both adsorbents and was used for further study. Above this mass, insignificant uptake of Tatz was observed. Fig. 11, increased uptake capacity of KMC and KMCM for Tatz were observed with an increase in the initial Tatz concentration. This could be due to the decrease of the resistance to mass transfer with increased Tatz concentration. At high concentrations of Tatz, the Tatz molecules are readily available at the binding sites and pores of KMC and KMCM. The adsorption potential of the unused adsorbents KMC was noticed to increase from 10 to 50 mg dm −3 above which a gradual increase is observed. Meanwhile, the nanocomposite showed a consistent increase in its uptake capacity with increased Tatz concentration. This suggests that the surface of KMCM consists of sufficient active sites that are capable of eliminating high Tatz concentrations in the aquatic ecosystem. To investigate the implication of solution temperature on the elimination of Tatz from the aquatic ecosystem, the effect of the initial concentration of Tatz experiment was repeated at different temperatures (298, 303, 308 and 313 K). The uptake capacity of the adsorbents was slightly enhanced with an increase in solution temperature, this suggests that the performance of the adsorbents was temperature dependent, also these adsorbents can be effective at elevated solution temperature. The equilibrium process involved in the uptake of the Tatz molecule from the aqueous phase can be used to investigate the pattern of adsorbent-adsorbate interactions. Meanwhile, in the design of a large-scale adsorption process, an understanding of the equilibrium process guides the optimization of the adsorbent 73 . The Redlich-Peterson models, Freundlich, Sips, Temkin, Dubinin Radushevick, Khan, Toht, and Langmuir isotherm analysis were all used in this work.

Effect of solution temperature and initial Tatz concentration. As displayed in
Data from experiments on the impact of the starting concentration and solution temperature experiment was fitted into the aforementioned isotherm models via the nls nonlinear regression routine. Models with the least SSR and RSE values were assumed to have the capacity to best describe the experimental data. Among the two-parameter models, the Langmuir adsorption isotherm was observed to best explain the experimental results, this indicates a monolayer coverage of KMC and KMCM. The maximum monolayer covering capacity (q m ) was observed to vary from 24.31 to 37.96 mg g −1 and 37.66 to 67.17 mg g −1 for KMC and KMCM respectively. A comparison of the monolayer adsorption capacity acquired for KMC and KMCM with other adsorbents showed the superior nature of KMCM (see Table 3). Meanwhile, the Langmuir adsorption constant (b), of KMCM was noticed to be higher than KMC (see Tables 4 and 5). This shows that the modification of this mineral (microcline) with f-MWCNTs did enhance the adsorptive force of the nanocomposite with increased solution temperature, hence, an increase in the sorbate to sorbent ratio of the adsorbents was observed. Amongst the three-parameter isotherms, the Sips model was noticed to best fit the experimental data. The Sips model also revealed the little contribution of heterogenous coverage from the non-unit n values estimated over the range of temperature investigated (see Tables 4 and 5).
Thermodynamic parameters of adsorption. Entropy change (∆S°), enthalpy change (∆H°) and free energy change (∆G°) are cardinal thermodynamic parameters that depend on the temperature of the adsorption process 74 . These parameters give insight into the implication of temperature on the sorbent-sorbate interaction, (1) www.nature.com/scientificreports/ T is the temperature in Kelvin, K L is the corrected dimensionless constant calculated from the product of the Langmuir constants q m , 1000 and b in dm 3 mol −176 , and R is the universal gas constant (8.314 J mol −1 K −1 ). At all temperatures studied, the ΔG° were negative (see Table 6), this suggests that the removal process is spontaneous  Adsorption mechanism. The choice of adsorbents in water decontamination via the batch adsorption technique is a function of the adsorbate chemistry as well as the surface characteristics of the adsorbent. The aforementioned determines the nature of sorbate-sorbent interaction. To verify the adsorptive mechanism responsible for the sequestration of tartrazine by KMCM, spectroscopic techniques (SEM and FTIR) and factors of adsorption experiments (solution pH and contact time) were employed. Deduction from the SEM analysis revealed the fixation of Tatz to the surface of KMCM. Meanwhile, FTIR spectra acquired for Tatz-KMCM showed decreased peak intensity and shift in adsorption bands. This indicates the possibility of chemisorption, which was further justified using the kinetic model assessment. Elovich kinetic model was noticed to describe the uptake of Tatz onto KMCM best. On the other hand, optimum adsorption by KMCM was achieved in the acidic medium (pH = 2). Hence, the Na atoms of tartrazine dye may be substituted by proton to form 3-carboxy-5-hydroxy-1-p-sulfophenyl-4-psulfophenylazopyrazole and can also protonate the nitrogen atoms of tartrazine. In summary, we propose that these charged sites on the adsorbate may bind with the hydroxyl functional groups on the surface of KMCM via electrostatic interaction (see Fig. 12). Owing to the hydrophobic nature of the modifier (MWCNTs) that was used for the adsorbent fabrication and the overall functional group on the adsorbent surface, the contribution of Van der Waals force hydrogen bonding, π-π stacking interaction in the removal of Tatz is certain 84,85 . Reusability. The recyclability of an adsorbent gives insight into the economic benefit of the material. The reusability of KMC and KMCM was investigated by performing adsorption followed by desorption. The reusability of KMC and KMCM for Tatz uptake was examined using NaOH as an eluting agent. As demonstrated in Fig. 13, after the fifth cycle, KMC and KMCM exhibited efficiency levels of approximately 30% and 71%, respectively. The reduced efficiency with increased usage may be attributed to the loss of binding sites. Hence, the nanocomposite has shown the capacity for industrial wastewater treatment practice.

Conclusion
In summary, we have successfully synthesized and characterized microcline-based nanocomposite for improved adsorption of tartrazine. FTIR analyses confirm the modification of microcline using MWCNTs and the incorporation of Tatz to the surface of KMC and KMCM. The uptake of Tatz using KMC or KMCM was noticed to be strongly dependent on solution pH, initial Tatz concentration and adsorbent dose. The acidic pH is suitable for the adsorptive removal of Tatz by KMC and KMCM. Meanwhile, optimum adsorptive conditions such as pH 2, 0.05 g adsorbent dose, 180 min contact time and 100 mg dm −3 initial concentration were established. The Langmuir isotherm model was found to best reflect the elimination of Tatz from KMC and KMCM. Meanwhile, the maximal monolayer uptake capacities of KMC and KMCM were determined to be 37.96 mg g −1 and 67.17 mg g −1 , respectively. The uptake kinetics shows that the pseudo-second-order and Elovich models express well the adsorption of Tatz onto KMC or KMCM respectively. Hence, it is inferred that KMC and KMCM were found to be promising for the effective removal of Tatz from the aquatic ecosystem.